import java.util.*;
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int m = in.nextInt();
        int[] step = new int[m + 1];
        for (int i = 0; i < m + 1; i++) {
            step[i] = Integer.MAX_VALUE;
        }
        step[n] = 0;
        for (int i = n; i < m; i++) {
            if (step[i] == Integer.MAX_VALUE) continue;
            List<Integer> list = div(i);
            for (int j : list) {
                if (i + j <= m && step[i + j] != Integer.MAX_VALUE) {
                    step[i+j]=Math.min(step[i+j],step[i]+1);
                } else if (i + j <= m) {
                    step[i+j] = step[i]+1;
                }
            }
        }
        if(step[m] == Integer.MAX_VALUE){
            System.out.print(-1);
        }else{
            System.out.print(step[m]);
        }
    }

    public static List<Integer> div(int num) {
        List<Integer> list = new ArrayList<>();
        for (int i = 2; i * i <= num; i++) {
            //if (num % i == 0) {
            //    list.add(i);
            //}
            if(num % i == 0) {
                list.add(i);
                if(num / i != i) {
                    list.add(num / i);
                }
            }
        }
        return list;
    }
}



